Suggest Remedy For Lightheadedness With Weakness And Sweating

Hi Im a 36 year old male and weight 300lbs or 21st roughly today I was gardening for 3 hours or so cutting down bushes and I experienced lightheadness and a weak feeling. This happens everytime I undertake some pyshical activity. I have visited my GP and he chalks it up to being overwieght. The thing is today my BP was 175/85 and my pulse was 105. However within an hour it had stabilied at 140/83 with a pulse 69 ( have a BP Monitor). The thing is this knocking my confidence I recongnise that I need to loss weigth but to exercise with this happens puts me off. On the other side of the coin I pushed out my little girl in her buggy for 2 miles on Thursday I came back sweaty but was fine. Sometimes I feel I need to weigh 15st before I am heard as fat people dont get ill. Can you offer any suggestions Richard